I am so glad to have found this... .trust me you are not the only woman with this problem!  I am 27 and have been experiencing the very same problem for the last few years.  During and immediately after orgasm, from intercourse or masturbation, I get  body and joint aches, neck stiffness, throbbing headache and nausea. This also happens when I perform oral sex on my husband, that does not result in an orgasm on my end.   It's the worst for several hours but the flu like symptoms will last for days. I have been gluten and dairy free for 4 years now, so it isn't allergies.  I have recently been diagnosed with fibromyalgia  and a tentative  MS dx, but still need a spinal tap.  Do you have any other illness, nerve pain, or odd symptoms..... do you get the same feeling with intense exercise?  I exercise frequently but get a similar headache only when I go running for some reason.. minus the flu symptoms though.  I have a normal body weight and BMI, and have normal blood pressure and blood work.  My doc has be taking 400 mg of riboflavin daily, which is apparently for migraines, although I never really get headaches at any other time